A hypothetical: a fully articulated _Megalosaurus_ is found - it has all the bits and peices used as the type specimen, but it also includes the rest of the skeleton, and the rest of the skeleton somehow shows it to be clearly not a descendant of the most recent common ancestor of _Triceratops_ and modern birds... yet _Megalosaurus_ was one of the two animals originally used to define Dinosauria...
rather unlikely....
just a hypothetical situation designed to illustrate the point.
(thank you for the explanation, btw.)
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